The Scorpion Mystery

In response to:

As a big fan of Tim Flannery’s work I was dismayed to see a set of quite incredible claims about scorpion fluorescence in his article “When a Scorpion Meets a Scorpion” [NYR, March 23]:

It’s difficult for human beings to see scorpions at night, but it’s easy for scorpions to see other scorpions. That is because scorpions produce bright green fluorescent light, which is clearly visible to them but invisible to the human eye.

As a physicist working in fluorescence applications in biomedical research and as an occasional scorpion hunter, I have experience across the relevant areas, and there are several points that just don’t add up:

There is no such thing as “bright green fluorescent light” that is “invisible to the human eye.” The three human visual pigments have broad and overlapping absorbance ranges so that there are no gaps in human color vision between the ultraviolet and the infrared. In fact, we see all shades of green with nearly our peak efficiency.

By personal observation, scorpion fluorescence is perfectly visible. Scorpion researchers and nature lovers like me find scorpions at night by carrying an ultraviolet lamp (blacklight) and looking for their “bright green fluorescent light.”

Fluorescent materials, such as scorpion cuticle, emit light only when excited by externally applied light at a shorter wavelength than the emission (e.g., ultraviolet excitation producing green emission). In the dark, a scorpion will produce no light and, therefore, not be visible to anyone, including another scorpion.

David Parks

Department of Genetics

Stanford University

Stanford, California

Tim Flannery replies:

I thank Professor Parks for his letter disputing two claims made in my recent review: that humans cannot see scorpion fluorescence, and that scorpions can see their own fluorescence. I should have specified that humans cannot see scorpion fluorescence unaided—i.e., without an ultraviolet lamp. That might be rather like saying that humans cannot fly without the assistance of an airplane, but sometimes we should state the obvious.

Things become a little murkier when we consider what scorpions can and cannot see. Professor Parks correctly notes that in the dark a scorpion will produce no fluorescent light. Yet most nights are not entirely dark, and I had assumed from reading David Attenborough’s Life in the Undergrowth that scorpions had sufficient night vision to detect each other’s fluorescence by the light of the heavens.

Professor Parks is an expert in this field, and he has conducted research showing that scorpion fluorescence is not very efficient. He also doubts that scorpion eyes function well in dim light, though here the data is less conclusive.

If Professor Parks is correct, scorpions cannot detect their own fluorescence, which begs the question why scorpions fluoresce at all. Could it be a warning to potential predators with better night vision? Or is it an evolutionary oddity with no function at all?

We are still learning the most astonishing things about animal communication, which leads me to wonder whether the mystery of the fluorescing scorpion is a tale for which the scientific world is not yet sufficiently prepared.